CLI tricks: Creating image thumbnails

If you publish content on the web you need to create thumbnails of published images every now and then. GIMP is useful for that, but why use such a heavy tool for such a simple task? It always took me about a minute to actually launch GIMP, do all the clicking to get a small version of the image and save it to another file. I have been doing it this way for ages, until I have heard of ImageMagick for the first time.